Simplify the styleResolverThrowAwayTimer

Around 1% of the time during style recalc is spent inside WTF::currentTime()
inside didAccessStyleResolver. This patch changes that method to be a simple
inline increment of a value and simplifies the logic for throwing away the
resolver.

Now every 30 seconds we'll look to see if any style recalc has happened, and
if none has discard the resolver. 

BUG=295262
R=ojan@chromium.org,eseidel@chromium.org

Review URL: https://chromiumcodereview.appspot.com/24249005

git-svn-id: svn://svn.chromium.org/blink/trunk@158407 bbb929c8-8fbe-4397-9dbb-9b2b20218538